1. Field of the Invention
The present invention relates to a sheet detecting apparatus which detects a sheet to be conveyed and an image forming apparatus including the sheet detecting apparatus.
2. Description of the Related Art
In general, a sheet conveying portion of an image forming apparatus includes a sheet detecting apparatus which detects a position of a leading edge of a sheet so as to match a timing for sending the sheet to a transfer position with a timing for sending an image formed by an image forming portion to the transfer position (see Japanese Patent Application Laid-Open No. H09-183539).
FIGS. 23 to 24C illustrate a conventional sheet detecting apparatus. As illustrated in FIGS. 23 and 24A, the conventional sheet detecting apparatus is provided on a downstream side in a sheet conveying direction of conveying roller pairs 518, 519 closest to a transfer position to which an image formed in an image forming portion is to be transferred. The sheet detecting apparatus includes a sensor lever 523 which abuts against a sheet S, a detection sensor 524, a light-shielding portion 525 which shields an optical path from a light-emitting portion to a light-receiving portion of the detection sensor 524 from light, and a stopper portion 526 which positions the sensor lever 523 at a waiting position. The sensor lever 523 is configured so as to be rotatable about a rotary shaft 527 and return to a waiting position due to a pressure force of a return spring 528 after the rotation. The light-shielding portion 525 is formed integrally with the sensor lever 523 and rotates together with the sensor lever 523.
As illustrated in FIG. 24A, when the leading edge of the sheet S comes into contact with the sensor lever 523, the sensor lever 523 rotates in a direction indicated by the arrow in FIG. 24A about the rotary shaft 527 from the waiting position and the light-shielding portion 525 shields the optical path of the detection sensor 524 from light. When the detection sensor 524 detects that the optical path has been shielded from light, the sheet detecting apparatus recognizes that the leading edge of the sheet S has reached the sensor lever 523. After that, the sheet S moves while being in contact with the leading edge of the sensor lever 523. When a trailing edge of the sheet S is separated from the sensor lever 523, the sensor lever 523 is rotated by the return spring 528 in a direction indicated by the arrow in FIG. 24C to return to the waiting position. At this time, the light-shielding portion 525 retreats from the optical path, and the light-receiving portion of the detection sensor 524 receives light from the light-emitting portion again, with the result that the sheet detecting apparatus recognizes that the trailing edge of the sheet S has passed by the sensor lever 523. By the way, in recent years, an image forming apparatus has been requested by users to have further enhanced throughput. In order to enhance throughput in the image forming apparatus, it is necessary to increase a conveying speed of a sheet and to reduce an interval from the trailing edge of a preceding sheet to the leading edge of a succeeding sheet (hereinafter, referred to as “sheet-to-sheet distance”). Therefore, the sheet detecting apparatus needs to return the sensor lever 523 to the waiting position within a short sheet-to-sheet distance after the passage of the preceding sheet S.
On the other hand, the conventional sensor lever 523 is configured in such a manner as to be pressed by the sheet S to rotate when the leading edge of the sheet S having passed by the conveying roller pairs 518, 519 abuts against an abutment portion and to rotate reversely to return to the waiting position when the trailing edge of the sheet S is separated from the abutment portion. Therefore, the distance required as a sheet-to-sheet distance is obtained by summing up a distance D1 from a position in which the trailing edge of a preceding sheet passes by the abutment portion of the sensor lever 523 to the waiting position in which the leading edge of a succeeding sheet abuts against the abutment portion and a distance D2 through which a succeeding sheet is conveyed during that time (see FIG. 24B).
Herein, the distance D2 is a distance (ΔT×V) obtained by multiplying a time ΔT during which the sensor lever 523 moves the distance D1 by a sheet conveying speed V. In the case where the sensor lever 523 reciprocates, the distance D1 for the sensor lever 523 to return to the waiting position is generated, and the distance D2 through which the succeeding sheet S is conveyed during the return operation of the sensor lever 523 becomes longer as the sheet conveying speed is higher. Therefore, the conventional sheet detecting apparatus has a problem in that a sheet-to-sheet distance becomes longer when the conveying speed of the sheet S is increased, which prevents the further enhancement of throughput.